Compensating network for binaural pickup



Nqv. 1, 1960 R GRAY 2,958,738

COMPENSATING NETWORK FOR BINAURAL PICKUP Filed Jan. 6, 1958 2 Sheets-Sheet l 5 IN V EN TOR.

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Nov. 1, 1960 R. B. GRAY 2,

COMPENSATING NETWORK FOR BINAURAL PICKUP Filed Jan. 6, 1958 2 Sheets-Sheet 2 CONIPENSATIN G NETWORK FOR BINAURAL PICKUP Robert B. Gray, Erie, Pa., assignor to Erie Resistor Corporation, Erie, Pa., a corporation of Pennsylvania Filed Jan. '6, 1958, Ser. No. 707,400

'5 Claims. (Cl. 179--100.4)

In binaural recordings, the inputs from the two binaural microphones are simultaneously recorded in a single groove with the respective excursions at right angles to each other. In order to reconstitute the sound, a pickup is used having independent response to the respective excursions and each response is fed through a separate channel to one of a pair of spaced or binaural speakers so that the outputs of the speakers give the impression of stereophonic or binaural sound. If the pickup is perfectly aligned, the binaural speakers reproduce the inputs to the binaural microphones. But, if the pickup should be tilted slightly from the position of perfect alignment, then there would be cross coupling so that each of the speakers would include a slight amount of the sound intendedfor the other. Cross coupling may also arise if the pickup itself is out of alignment. This invention is intended to introduce compensation for such misalignment which may be expected to some extent as a result of the inevitable variations in manufacture.

In the drawing, Fig. 1 is an end view of a binaural pickup element; Fig. 2 is a side view of the pickup element and needle assembly; Fig. 3 is a diagram illustrating the response when the pickup element is tilted; and Figs. 4, 5, and 6 are circuit diagrams, each including a network compensating for tilting or misalignment of the pickup element.

In Figs. 1 and 2, there is shown the operating elements of a binaural pickup consisting of an elongated bar 1 of barium titanate or other polarizable piezoelectric ceramic having pairs of electrodes 2, 3 and 4, with the pair of electrodes 2, 3 on an axis at right angles to the pair of electrodes 4, 5. The polarization of each pair of electrodes is opposite so that each pair will respond to bending in an axis in a plane midway between the electrodes. One way of achieving the opposite polarization is to connect the polarizing voltage between adjacent electrodes thereby producing the electrostatic field distribution or polarization indicated by the arrows 6. In Fig. 1, the ceramic element 1 is shown of square section. Other shapes may be used; for example, those disclosed in my application Serial No. 516,638, filed June 20, 1955. In order that the element may respond independently to bending forces about axes in planes at right angles to each other, it is necessary that in each pair of electrodes, the polarization adjacent one electrode be opposite to the polarization adjacent the other electrode of the pair and that the pair of electrodes of one pair be symmetrically disposed with respect to an axis at right angles to the other pair. These requirements are fulfilled by the square cross section element illustrated in Fig. l and by the other shapes illustrated in my aforesaid application.

In the use of the pickup element, one end 7 is fixed in the end of a chuck 8, for example by cementing, and the other end 9 is suitably fixed to a cartridge structure. The chuck 8 carries at its free end a needle 10, which as it follows the record groove, is moved by excursions in the directions of arrows 11 and 12. The arrows 11 and 12 are at right angles to each other with the arrow 11 per- States Patent ice pendicular to the pair of electrodes 2, 3 and the arrow 12 perpendicular to the pair of electrodes 4, 5. The excursions in the direction of the arrow 11 cause bending of the element about an axis midway between the electrodes 2, 3 and perpendicular to the electrodes 4, 5. This bending causes a voltage to appear in the electrodes 2, 3 proportional to the magnitude of the excursion in the direction of the arrow 11 and this voltage appears across the leads 13 and 14 connected to the electrodes 2 and 3. The excursion in the direction of the arrow 12 causes bending of the ceramic element about an axis midway between the electrodes 4 and 5 and perpendicular to the electrodes 2 and 3, thereby producing a voltage proportional to the magnitude of the excursion in the direction of arrow 12, which appears in leads 15 and 16 connected to the electrodes 4 and 5. By feeding the output from the leads 13, 14 and 15, 16 through separate channels to separate speakers, the binaural input to the record groove represented by the arrows 11 and 12 is reconstituted and recreates the same stereophonic or binaural sound effect picked up by the separate binaural microphones which recorded the excursions corresponding to the arrows 11 and 12 in the record groove.

Due to variations in manufacture, it frequently happens that the pickup is tilted so that it is not in precise alignment with the record excursions indicated by the arrows 11 and 12. This is illustrated in Fig. 3 where the axis X indicates the axis of the record excursion of arrow 11 which is designated as X sin w t and where the axis Y indicates the axis of the excursion of arrow 12 designated as Y sin 111- 1. The pickup is shown tilted at an angle A in the sense that the axes U and V orthogonal to the electrodes 2, 3 and 4, 5 make an angle A with respect to the axes X and Y. If the axes U and V were in perfect alignment with the axes X and Y, then the output appearing across the electrodes 2, 3 and 4, 5 would be exactly proportional to the respective inputs of the binaural microphones and the binaural sound would be perfectly reconstituted. However, when the pickup is tilted, there .is cross coupling in the sense that some of the record excursion in the direction of the arrow 11 appears across the electrodes 4 and 5 and some of the record excursion in the direction of the arrow 12 appears across the electrodes 2 and 3. If the output appearing across the elec-' trodes 2 and 3 is designated as U and the output appearing across the electrodes 4 and 5 is designated as V, then the outputs will be given by the following equations:

U=K X cos A sin w t+K Y sin A sin w t V=K X sin A sin w t-l-K Y cos A sin Wgt In the foregoing equations, K and K are the electromechanical coupling coefficients, which should be equal to each other, if the ceramic bar 1 is perfectly symmetrical.

It will be noted from these equations that while the major term in the output appearing across the electrodes 2, 3 and 4, 5 corresponds to the input excursions, there is a minor term representing the unwanted cross coupling. For example, if there were no cross coupling, U would contain only frequencies sin W and V would contain only frequencies sin w t. If a percentage of V is substracted from U and a percentage of U is substracted from V, the unwanted cross coupling can be eliminated.

One way of accomplishing this substraction is by the circuit shown in Fig. 4 where the electrodes 2 and 5 are connected to ground through an adjustable resistance 17 and the electrodes 3 and 4 are respectively connected to ground through separate adjustable resistances 18 and 19. The output voltage appears across resistances 18 and 19 and is fed through separate amplifiers 20 and 21 feeding separate binaural speakers 22 and 23. Because of the common resistance 17, the voltage appearing across the resistances 18 and 19 is less than the voltage outputs appearing across the electrodes 2, 3 and 4, by the amount of the voltage drop across the adjustable resistance 17. Considering the output across resistance 19, it is apparent that that voltage will be less than the voltage appearing across electrodes 2 and 3 by an amount which includes a fraction of the voltage output appearing across electrodes 4 and S as well as the voltage division between resistors 17 and 19. A similar result is obtained with the voltage output appearing across resistance 13, which is decreased by a fraction of the voltage appearing across electrodes 2 and 3 as well as the voltage division between resistor 17 and 18. This circuit accordingly provides the necessary subtraction so that the resultant voltages to be amplified, which appear respectively across the resistances l3 and 19, are corrected for the cross coupling due to misalignment of the character illustrated diagrammatically in Fig. 3. Ordinarily, the misalignment will not be more than five degrees and the amount of subtraction necessary to eliminate the cross coupling will be small. There is some loss in output, due to the subtraction, but even if the maximum misalignment is ten degrees, the loss in output is less than six percent due to the correction term. However, for this misalignment of ten degrees, there would be a cross coupling of nearly twenty percent, so that the voltage division loss would be nearly twenty percent, making a total of nearly 26% for this circuit.

Fig. 5 shows another circuit in which the same compensation for cross coupling is obtained without the decrease in output present in Fig. 4 due to the voltage division. Fig. 4, it will be noted that the electrodes 2 and 5, which had the same instantaneous potential, for example negative, were connected to the resistor 12 and the outputs were taken from the electrodes 3 and 4, which instantaneously would have a potential opposite that appearing on the electrodes 2 and 5. In the circuit of Fig. 5, the electrodes 2 and 4 which have an instantaneous output voltage opposite each other are connected to the adjustable resistance 17a, which corresponds to the resistance 17 in Fig. 4 and the output is taken from electrodes 3 and 5 which are respectively connected to adjustable resistances 18a and 19a corresponding to the resistances 18 and 19 in Fig. 4. It will be noted that so far as the adjustable resistance 17a is concerned, the voltages from the respective pairs of electrodes 2, 3 and 4, 5 are connected in opposite sense so that there is a substraction which takes place in the adjustable resistance 17a due to the circuit connections. With the Fig. 5 circuit, the full voltage appearing across electrodes 2 and 3 is fed to the amplifier 21a and speaker 23a and the full voltage appearing across electrodes 4 and 5 is fed in a negative sense to amplifier 263a and speaker 22a. As in Fig. 4, there is a subtraction to compensate for cross coupling, but reduction in the output is limited to that required for correction, or about 6% for a angle error, rather than the 26% of the previous circuit.

The circuit of Fig. 5 may be extended to push-pull amplifiers, as shown in Fig. 6, where electrodes 2 and 4 which have instantaneous voltages in opposite sense are connected to adjustable resistances 19b and 18b which are connected to ground through an adjustable resistance 17b and the electrodes 3 and 5 which also have instan taneous voltage of opposite sense are connected to adjustable resistances 19c and 180 which are connected to ground through a common adjustable resistance 17c. The resultant output from electrodes 4 and 5 appears across resistors 18b and 180 and is connected to pushpull amplifier 20b feeding speaker 22b. The output from electrodes 2 and 3 appears across resistances 1% and 190 and is fed to push-pull amplifier 21b feeding speaker 23b. The resistances bearing the subscripts b and c in Fig. 6 are equivalent in function to the conespondingly numbered resistances in Fig. 4 and Fig. 5. Because the voltages from the respective pairs of elec- 4 trodes 2, 3 and 4, 5 flow through the resistances 17b and 17c in opposite sense, there is the necessary subtraction in order to remove the objectionable cross coupling.

It will be noted that the resistances 18 and 19 as well as the corresponding resistances bearing the subscripts a, b, and c are shown to be adjustable. If the pickup element were perfectly symmetrical, these resistances would be fixed resistances because the output for a given deflection appearing across the electrodes 2, 3 would be identical with the output appearing across the electrodes 4, 5. The adjustment of the resistances l8 and 19 as well as the corresponding resistances bearing the subscripts a, b, or c, is for the purpose of compensating for differences in output of the pickups due to variations in manufacture. The resistances 18 and 19 can be regarded as compensating for variations in output of the pickups, while the resistance 17 can be regarded as compensating for misalignment of the mounting of the pickup in its cartridge.

While the excursions in the record groove corresponding to the binaural inputs indicated by the arrows l1 and 12 are at right angles to each other, it will be appreciated that the excursion corresponding to the arrow 11 need not be horizontal and the excursion corresponding to the arrow 12 need not be vertical. Both excursions may be at an angle of forty-five degrees to the horizontal. The important factor is that the excursions are at right angles to each other.

From one aspect, each output of the pickup is fed to voltage dividers consisting of a larger resistance and a smaller resistance with the larger resistances independent of each other. The smaller resistance is adjusted to determine the fraction of signal from one output to be coupled in a subtractive sense into the other output. The larger of the resistances is adjustable to compensate for differences in signal response of the outputs of the pickup. The smaller resistance is shown common to both dividers, but the same function could be obtained with separate resistances.

Furthermore, it is possible to use any type of impedance to create these correction circuits. Resistances have been shown as the most usual ones because they provide a direct current path suitable for a tube grid input, and also are readily available in adjustable form. However inductances could readily be used. Or again, capacity elements, or any combination which might be desired to provide a tone control or frequency compensating network.

What is claimed as new is:

1. In a binaural phonograph system for records in which each groove has excursions at right angles to each other and respectively from one and the other of two binaural microphones, a pickup having a needle simultaneously following both excursions and having means moved by the needle generating separate signals responsive to movements of the needle in directions at right angles to each other, a separate output for each signal, and a separate amplifying channel for each signal respectively feeding one and the other of two binaural speakers to reconstitute the sound from the binaural microphones, the improvement which comprises means for cross coupling from each channel a fraction of its own signal to the other channel in a subtractive sense to the signal in said other channel, and means for adjusting the amount of cross coupling to compensate for misalignment of the pickup with respect to the excursions in the record groove.

2. The system of claim 1 having means for independently adjusting the fraction of cross coupling of each channel to further compensate for differences in response of the pickup in. said directions at right angles to each other.

3. The system of claim 1 in which each output includes a voltage divider across which the signal to be fed from its channel appears and the cross coupling is obtained by connecting the voltage dividers to each other at a point corresponding to the desired fraction of cross coupling.

4. The system of claim 1 in which each output has a voltage divider comprising two resistances, one resistance in each divider being smaller than the other and adjustable, and the larger resistance of each divider being independent of the other divider and further in which the cross coupling voltage is obtained across the smaller of the resistances.

5. The system of claim 4 in which the larger of the resistances of each divider is adjustable to compensate for differences in the signal response in said outputs.

References Cited in the file of this patent UNITED STATES PATENTS Blumlein Sept. 21, Keller et al. Apr. 19, Guanella June 15, Singer Dec. 13, MacAdam Mar. 11, Bertram July 29, 

